The present invention relates to a semiconductor integrated circuit and an operating method thereof, and particularly to a technology effective in generating local signals supplied to a reception mixer without using a PLL circuit having a problem with responsivity.
A so-called IC card equipped therein with a semiconductor integrated circuit and an antenna performs exchange of information between a reader-writer device and the semiconductor integrated circuit and achieves various functions such as transmission of data retained in the IC card, holding of data transmitted from the reader-writer device, etc.
According to the international standard ISO/IEC14443, an IC card having a non-contact interface is called PICC and performs RF communications with a reader/writer device called PCD. Incidentally, ISO is an abbreviation of International Organization for Standardization. IEC is an abbreviation of International Electrical Commission. PICC is an abbreviation of a Proximity Card, and PCD is an abbreviation of a Proximity Coupling Device.
For example, in the type A of the international standard ISO/IEC14443, the communication from the PCD to the PICC has been defined as an encoding system based on a deformation mirror system under a modulation scheme with a modulation depth of ASK100%. In the type B of the international standard ISO/IEC14443, however, the communication from the PCD to PICC has been defined as an encoding system based on an NRZ-L system under a modulation scheme with a modulation rate of ASK10%. Incidentally, NRZ-L is an abbreviation of Non Return to Zero-Level. ASK is Amplitude Shift Keying that is one of digital modulation schemes.
A communication technology has recently been in widespread use, which is called a near field communication (NFC) technology and which simplifies wireless communication couplings to household electrical appliances, digital media and consumers, contents and business transactions and spreads them. This NFC technology has compatibility with the existing various communication systems and enables near field communication of the maximum communication rate 847 Kbps at 10 cm or so using an RF frequency of 13.56 MHz. In particular, a cellular phone terminal having built therein an IC card microcomputer (secure chip) having an electronic payment function is equipped with the NFC technology to aim at enhancing convenience for end users by making use thereof for various contactless electronic payments such as a payment for merchandise purchases at a contactless store, a payment for traveling expenses at a station, etc. Incidentally, NFC is an abbreviation of Near Field Communication.
The communication that utilizes ASK modulation between a contactless IC card using this NFC technology and a reader-writer device makes use of electromagnetic coupling between a relatively large loop antenna of the reader-writer device and a relatively small loop antenna of the contacless IC card.
On the other hand, the following Non-Patent Document 1 has described that a communication hole occurs due to the angles of loop antennas of a contacless IC card and a reader-writer device, the distance between them, etc. This phenomenon shows that ASK-modulated amplitude at load modulation is almost unobtainable though the contactless IC card exists within an operating magnetic field range of the reader-writer device, and sufficient RF power is supplied from the reader-writer device to the contactless IC card.
The cause of occurrence of the communication hole results from the fact that when the loop antenna of the contacless IC card is disposed approximately parallel to the peripheral portion of the large loop antenna of the reader-writer device, the directions of two lines of magnetic field generated from the inside and outside of the loop antenna of the reader-writer device are opposite inside the loop antenna of the contactless IC card to thereby cancel out an RF reception signal. Further, the following Non-Patent Document 1 also has described a proposal in which a synthetic vector containing not only the amplitude of a real number but also the amplitude and phase in the direction of an imaginary axis is provided as a solution to the communication hole.
Further, the following Non-Patent Document 2 has described a null-point avoidance circuit that avoids a null point that a detected signal is not obtained due to the relationship of phase between a received signal and local signal (LO) waves where homodyne detection is applied to a reader-writer device for an RFID system. The null-point avoidance circuit detects the received signal by the LO waves different in phase and selectively receives a channel large in detection level. Further, in the null-point avoidance circuit, an RF received signal is supplied to an input terminal of one of two reception mixers, and an LO signal and a phase-shifted LO signal phase-shifted by π/2(90)° are respectively supplied to the other input terminal of one reception mixer and the other input terminal of the other reception mixer. An I channel baseband signal from the output of one reception mixer, and a Q channel baseband signal from the output of the other reception mixer are respectively supplied to one input terminal of a selector and the other input terminal thereof through an I channel baseband amplifier and a Q channel baseband amplifier. A baseband signal having a large detected level of either one of an I channel and a Q channel is selected as reception data by the selector and supplied to a control unit. Incidentally, the null point is considered to be a phenomenon substantially similar to the above communication hole.
A direct conversion receiver that executes the same receiving operation as the homodyne receiver described in the Non-Patent Document 2 in principle has been described in the following Patent Document 1. Local signals supplied to two reception mixers and phase-shifted local signals different in phase by 90° are generated from phase-locked loop (PLL) including a voltage-controlled oscillator (VCO). A traditional superheterodyne receiver needs an image elimination filter for interference-wave elimination of an image frequency with frequency conversion, whereas the direct conversion receiver described in the following Patent Document 1 has an advantage that an image elimination filter used as an external component becomes unnecessary.
Further, a direct conversion receiver used in a digital cordless phone has been described in the following Non-Patent Document 3. The Non-Patent Document 3 also has described that a double balance type cross-coupled quad mixer called a Gilbert mixer is generally used for a reception mixer of the direct conversion receiver. In this reception mixer, an analog circuit configuration has been adopted in which a load resistor, two differential pair transistors and a constant current source are coupled in series between a power supply voltage and a ground voltage.    [Patent Document 1] Japanese Unexamined Patent Publication No. 2001-211098    [Non-Patent Document 1]    Hiroshi Karibe “Introduction To Design of Contactless IC Card”, Published by Nikkan Kogyo Shinbun Ltd., PP 46-50, Oct. 31, 2005.    [Non-Patent Document 2]    Naohisa Takayama “Effects of Null-Point Avoidance Circuit for UHF-Band RFID Reader-Writer Device”, 2005 IEICE Communication Society Conference, Program Number B-5-166, Page 566.    [Non-Patent Document 3]    Christopher Dennis Hull, et al, “A Direct-Conversion Receiver for 900 MHz (ISM Band) Spread-Spectrum Digital Cordless Telephone”, IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 31, NO. 12, DECEMBER 1996, PP. 1955-1963.